90 GENERAL PRINCIPLES OF ZOOLOGY 



and forms the jaws, the hyoid bone, and the small bones of the ear, which 

 in spite of their quite different functions, are morphologically the same 

 structures as the gill-arches. 



Homology and Analogy. In the History of Zoology (page 10) 

 it was shown that comparative anatomy has caused a discrimination be- 

 tween homology or morphological equivalence, and analogy or physio- 

 logical equivalence, i.e., between organs which appear in the same relative 

 positions and relations, and organs which have the same function. What 

 we have here learned of the structure of organs makes it evident that 

 morphological and physiological characters do not necessarily coincide, 

 that morphologically similar organs (lungs of mammals, air bladder of 

 fishes) may have different functions, morphologically different organs 

 (lungs of mammals, gills of fishes) the same functions. 



Systems of Organs. Organs identical, or, at least, functioning in an 

 equivalent manner, may occur in considerable numbers in the same body. 

 A man has many muscles, and many organs which carry on digestion. 

 Hence we may group all organs which in the body have equivalent or 

 similar functions into systems of organs. In all we recognize nine such 

 systems: (i) skeletal, (2) digestive, (3) respiratory, (4) circulatory, (5) 

 excretory, (6) genital, (7) muscular, (8) nervous, and (9) sensory systems. 

 Not all are necessarily present; a skeleton, for instance, is frequently lack- 

 ing. The various functions, which in man are divided among different 

 complicated and specialized systems, may be performed in a lower animal 

 by one and the same apparatus. Yet according to the fundamental 

 functions the following groups of organs may be recognized: I. Organs 

 of assimilation (2-5); II. Organs of reproduction (6); III. Organs of 

 motion (7); IV. Organs of perception (8 and 9). 



Vegetative and Animal Organs. The organs of assimilation and of repro- 

 duction (I and II) are grouped together as vegetative, the others (III and IV) 

 as animal organs. The older zoologists used to say that assimilation and repro- 

 duction are functions common to animals and plants; that, on the contrary, 

 sensation and motion are lacking in plants, and are exclusively characteristic 

 of animals. The atom of truth in the fundamental idea needs reconsideration 

 in the light of our present knowledge. We have seen that the protoplasm of 

 plants and animals has not only the power of assimilation and reproduction, 

 but also power of motion and of irritability. The latter characteristics conse- 

 quently cannot be entirely lacking in all the plants, for they are found in their 

 most important constituent. In fact many plants, as the mimosas, compass- 

 plants, insectivorous plants, show great irritability; the reproductive states of 

 algae move quite as actively as, or even more actively than, many of the lower 

 animals. On the other hand, many animals are fixed in position like plants. 

 Many Protozoa and worms, most polyps, some echinoderms like the crinoids, 

 even many Crustacea, the cirripedes (barnacles), can change their location only 

 during the earlier stages of development, in later life being limited to movements 

 of single parts of the body, the arms, tentacles, etc. In the sponges motions are 



